The present invention relates to superluminescent diodes (hereinafter referred to as "SLDs") and a method for manufacturing the same. More particularly, it relates to an SLD capable of emitting a high intensity of incoherent light with a small radiation angle, which is useful as a light source for optical fiber gyros, optical sensors, optical disks and the like, and to a method for manufacturing the same.
In an SLD, which emits incoherent light of a large output from an end face of an active layer thereof, it is of great importance that laser oscillation in Fabry-Perot (FP) mode is restrained. To this end, the following SLD structures have heretofore been proposed:
structure (1) wherein an antireflective (AR) coat 30 is formed on opposite end faces to reduce the reflectance thereby restraining FP mode oscillation (refer to FIG. 15); PA0 structure (2) wherein one half of an active layer is used as an non-excitation region 31 which is adapted to absorb light produced at a current-injection region so as to decrease the reflectance of an end face, thereby restraining FP mode oscillation (refer to FIG. 16); and PA0 structure (3) wherein light produced at a current-injection region is total-reflected by an end face using a bent waveguide 32 thereby restraining FP mode oscillation (refer to FIG. 17). In FIGS. 15 to 17, numeral 33 denotes a current-injection region, numeral 30 denotes an antireflective coat, and numeral 31 denotes a non-excitation region. PA0 (a) sequentially stacking on a semiconductor substrate of a conductivity type a lower cladding layer of the same conductivity type as that of the semiconductor substrate, an active layer of n-type or p-type conductivity or undoped, a first upper cladding layer of a conductivity type opposite to that of the semiconductor substrate, and a current-blocking layer of the same conductivity type as that of the semiconductor substrate; PA0 (b) forming current-injection regions each of a stripe-shaped groove shape by digging the wafer resulting from the step (a) down to the current-blocking layer with use of a mask having stripe-shaped groove openings arranged lengthwise and crosswise; PA0 (c) sequentially stacking on the wafer formed with the current-injection regions a second upper cladding layer and a cap layer, each being of a conductivity type opposite to that of the semiconductor substrate; PA0 (d) abrading the semiconductor substrate to a predetermined thickness; PA0 (e) forming ohmic electrodes on upper and lower surfaces of the wafer; PA0 (f) cutting the wafer into chips such that each chip includes a rear portion of one of two adjacent current-injection regions and a front portion of the other; and PA0 (g) forming a protective film or a coating film of a low-reflectance (or reflection) on opposite end faces, respectively, of each of the chips.
With the structure (1) having the AR coat on the end face, however, an AR coat having an extremely low reflectance for sufficiently restraining laser oscillation needs to be formed with a good reproducibility. This results in a difficulty in device fabrication.
With the structure (2) having the non-excitation region in one half of the active layer and the structure (3) using the bent waveguide, a light output can hardly be expected from the end face opposite to the end face for outputting light and, hence, automatic power control (APC) is hard to realize. Further, with the structure (2) the non-excitation region needs to be made long to sufficiently restrain FR mode oscillation because the non-excitation region is not provided with a special light-absorbing mechanism. Also with the structure (3), a long waveguide is needed for the bent waveguide. Thus, in either case there arises a problem of increased device size (especially in the longitudinal direction).